1. Field of the Invention
This invention generally relates to an apparatus for and a method of electro-optically reading indicia having parts of different light reflectivity, for example, bar code or matrix array symbols, and, more particularly, to controlling the viewing angle of, or the light input to, photodetectors used in such apparatus for sensing light reflected from such indicia.
2. Description of the Related Art
Various optical readers and optical scanning systems have been developed heretofore for reading indicia such as bar code symbols appearing on a label or on the surface of an article. The bar code symbol itself is a coded pattern of indicia comprised of a series of bars of various widths spaced apart from one another to bound spaces of various widths, the bars and spaces having different light reflecting characteristics. The readers in scanning systems electro-optically transform the graphic indicia into electrical signals, which are decoded into alphanumeric characters that are intended to be descriptive of the article or some characteristic thereof. Such characteristics are typically represented in digital form and utilized as an input to a data processing system for applications in point-of-sale processing, inventory control and the like. Scanning systems of this general type have been disclosed, for example, in U.S. Pat. Nos. 4,251,798; 4,369,361; 4,387,297; 4,409,470; 4,760,248; 4,896,026, all of which have been assigned to the same assignee as the instant application. As disclosed in some of the above patents, one embodiment of such a scanning system resides, inter alia, in a handheld, portable laser scanning device supported by a user, which is configured to allow the user to aim the scanning head of the device, and more particularly, a light beam, at a targeted symbol to be read.
The light source in a laser scanner bar code reader is typically a gas laser or semiconductor laser. The use of semiconductor devices as the light source is especially desirable because of their small size, low cost and low voltage requirements. The laser beam is optically modified, typically by an optical assembly, to form a beam spot of a certain size at the target distance. It is preferred that the cross section of the beam spot at the target distance be approximately the same as the minimum width between regions of different light reflectivity, i.e., the bars and spaces of the symbol. At least one bar code reader has been proposed with two light sources to produce two light beams of different frequency.
The bar code symbols are formed from bars or elements typically rectangular in shape with a variety of possible widths. The specific arrangement of elements defines the character represented according to a set of rules and definitions specified by the code or "symbology" used. The relative size of the bars and spaces is determined by the type of coding used as is the actual size of the bars and spaces. The number of characters (represented by the bar code symbol) is referred to as the density of the symbol. To encode the desired sequence of the characters, a collection of element arrangements are concatenated together to form the complete bar code symbol, with each character of the message being represented by its own corresponding group of elements. In some symbologies, a unique "start" and "stop" character is used to indicate when the bar code begins and ends. A number of different bar code symbologies exist, these symbologies include UPC/EAN, Code 39, Code 128, Codeabar, and Interleaved 2 of 5, etc.
In order to increase the amount of data that can be represented or stored on a given amount of surface area, several new bar code symbologies have recently been developed. One of these new code standards, Code 49, introduces a "two dimensional" concept by stacking rows of characters vertically instead of extending the bars horizontally. That is, there are several rows of bar and space patterns, instead of only one row. The structure of Code 49 is described in U.S. Pat. No. 4,794,239, which is herein incorporated by reference. Another two-dimensional symbology, known as "PDF417", is described in U.S. patent application, Ser. No. 07/461,881, now U.S. Pat. No. 5,304,786.
Still other symbologies have been developed in which the symbol is comprised of a matrix array made up of hexagonal, square, polygonal and/or other geometric shapes. Such symbols are further described in, for example, U.S. Pat. Nos. 5,276,315 and 4,794,239. Such matrix symbols may include Vericode, Datacode, and MAXICODE.
In the laser beam scanning systems known in the art, the laser light beam is directed by a lens or other optical components along the light path toward a target that includes a bar code symbol on the surface. The moving-beam scanner operates by repetitively scanning the light beam in a line or series of lines across the symbol by means of motion of a scanning component, such as the light source itself or a mirror disposed in the path of the light beam. The scanning component may either sweep the beam spot across the symbol and trace a scan line across the pattern of the symbol, or scan the field of view of the scanner, or do both.
In retroreflective light collection, a single optical component, e.g., a rotating mirror, as described in Krichever et al. U.S. Pat. No. 4,816,661 or Shepard et al. U.S. Pat. No. 4,409,470, both herein incorporated by reference, scans the beam across a target surface and directs the collected light to a detector. The rotating mirror must be relatively large to receive the incoming light, but only a small detector is required because the rotating mirror can focus the light on to a small area. As a result, the detector has a relatively small field of view, which increases signal-to-noise ratio.
In non-retroreflective light collection, the reflected laser light is not collected by the same rotating mirror used for scanning. Instead the detector is constructed to have a large field of view so that the reflected laser light traces across the surface of the detector. Because the rotating mirror need only handle the outgoing light beam, it can be made much smaller. But the detector must be relatively large in order to receive the incoming light beam from all scanned locations.
Bar code reading systems also include a sensor or photo-detector which detects light reflected or scattered from the symbol. The photo-detector or sensor is positioned in the scanner in an optical path so that it has a field of view which ensures the capture of a portion of the light which is reflected or scattered off the symbol. The captured light is detected and converted into an electrical signal. Electronic circuitry and software decode the electrical signal into a digital representation of the data represented by the symbol that has been scanned. For example, the analog electrical signal generated by the photodetector is converted by a digitizer into a pulse width modulated digitized signal, with the widths corresponding to the physical widths of the bars and spaces. Such a digitized signal is then decoded, based on the specific symbology used by the symbol, into a binary representation of the data encoded in the symbol, and subsequently to the alphanumeric characters so represented.
The decoding process of known bar code reading system usually works in the following way. The decoder receives the pulse width modulated digitized signal from the digitizer, and an algorithm, implemented in the software, attempts to decode the scan. If the start and stop characters and the characters between them in the scan were decoded successfully and completely, the decoding process terminates and an indicator of a successful read (such as a green light and/or an audible beep) is provided to the user. Otherwise, the decoder receives the next scan, performs another decode attempt on that scan, and so on, until a completely decoded scan is achieved or no more scans are available.
Such a signal is then decoded according to the specific symbology into a binary representation of the data encoded in the symbol, and to the alphanumeric characters so represented.
Moving-beam laser scanners are not the only type of optical instrument capable of reading bar code symbols. Another type of bar code reader is one which incorporates detectors based on charge coupled device (CCD) technology. In such prior art readers the sides of the detector are typically smaller than the symbol to be read because of the image reduction by the objective lens in front of the CCD. The entire symbol is flooded with light from a light source such as lighting light emitting diodes (LED) in the scanning device, and each CCD cell is sequentially read out to determine the presence of a bar or a space.
The working range of CCD bar code scanners is rather limited as compared to laser based scanners and is especially low for CCD based scanners with an LED illumination source. Other features of CCD based bar code scanners are set forth in U.S. patent application Ser. No. 08/041,281 which is hereby incorporated by reference, and in U.S. Pat. No. 5,210,398. These references are illustrative of the earlier technological techniques proposed for use in CCD type scanners to acquire and read indicia in which information is arranged in a two dimensional pattern.